Multi-sector back-off logic algorithm for obtaining optimal slice-sensitive computed tomography profiles

ABSTRACT

A multi-sector back-off logic algorithm for obtaining optimal slice-sensitive computed tomography (“CT”) profiles. The systems and methods of the present invention improving the temporal resolution of a CT system by checking for Z location errors between sectors and automatically backing-off to an alternative multi-sector algorithm when necessary (i.e., selecting an optimized maximum number of sectors to reconstruct), providing less Z location error. Based upon this Z location error, the systems and methods of the present invention also calculating the maximum number of sectors that should be used for reconstruction “on-the-fly” (i.e., on a per image basis across an entire series of images). These systems and methods utilizing the Recommended Protocol for Cardiac Reconstruction Algorithms.

FIELD OF THE INVENTION

The present invention relates generally to computed tomography (“CT”)systems and methods. More specifically, the present invention relates toa multi-sector back-off logic algorithm for obtaining optimalslice-sensitive CT profiles, especially for cardiac applications.

BACKGROUND OF THE INVENTION

Computed tomography (“CT”) systems are often used to image the heart andcardiovasculature. The data for a given image may be collected frommultiple cardiac cycles using multiple sectors. This creates a number ofchallenges. In an ideal case, the multiple sectors used to reconstructthe heart and cardiovasculature overlap for a zero Z location errorbetween sectors. This, however, is not always the case. For a relativelylow heart rate and high pitch, for example, the sectors used toreconstruct the heart and cardiovasculature do not always overlap,resulting in a relatively large Z location error between sectors andrelatively poor slice-sensitive profiles. Because of this, the datacollected from multiple cardiac cycles may be too far apart, resultingin poor image quality.

Thus, what is needed are systems and methods that generate high temporalresolution images for cardiac CT applications while addressing theproblem of bad images by checking for these Z location errors betweensectors and automatically backing-off to an alternative multi-sectoralgorithm when necessary (i.e., selecting an optimized maximum number ofsectors to reconstruct), providing less Z location error. What is alsoneeded are systems and methods that, based upon this Z location error,calculate the maximum number of sectors that should be used forreconstruction “on-the-fly” (i.e., on a per image basis across an entireseries of images). Preferably, these systems and methods utilize theRecommended Protocol for Cardiac Reconstruction Algorithms.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides a multi-sector back-offlogic algorithm for obtaining optimal slice-sensitive computedtomography (“CT”) profiles. The systems and methods of the presentinvention generate high temporal resolution images for cardiac CTapplications and address the problem of bad images by checking for Zlocation errors between sectors and automatically backing-off to analternative multi-sector algorithm when necessary (i.e., selecting anoptimized maximum number of sectors to reconstruct), providing less Zlocation error. Based upon this Z location error, the systems andmethods of the present invention also calculate the maximum number ofsectors that should be used for reconstruction “on-the-fly” (i.e., on aper image basis across an entire series of images). These systems andmethods utilize the Recommended Protocol for Cardiac ReconstructionAlgorithms.

In one embodiment of the present invention, a computed tomography methodincludes determining a maximum Z location error and determining aweighted average Z location error. The computed tomography method alsoincludes selecting a threshold value associated with the maximum Zlocation error and the weighted average Z location error. The computedtomography method further includes prescribing an N+1 sectorreconstruction algorithm. If the maximum Z location error is less thanor equal to the threshold value or the weighted average Z location erroris less than or equal to the threshold value, the computed tomographymethod includes performing an N+1 sector reconstruction. If the maximumZ location error exceeds the threshold value or the weighted average Zlocation error exceeds the threshold value, the computed tomographymethod includes prescribing an N sector reconstruction.

In another embodiment of the present invention, a computed tomographymethod for obtaining optimal slice-sensitive profiles includesdetermining a maximum Z location error associated with a computedtomography system and determining a weighted average Z location errorassociated with the computed tomography system. The computed tomographymethod also includes selecting a threshold value associated with themaximum Z location error and the weighted average Z location error. Thecomputed tomography method further includes prescribing an N+1 sectorreconstruction algorithm. If the maximum Z location error is less thanthe threshold value or the weighted average Z location error is lessthan the threshold value, the computed tomography method includesperforming an N+1 sector reconstruction. If the maximum Z location errorexceeds the threshold value or the weighted average Z location errorexceeds the threshold value, the computed tomography method includesprescribing an N sector reconstruction.

In an further embodiment of the present invention, an imaging method forobtaining optimal slice-sensitive profiles includes determining amaximum Z location error associated with an imaging system anddetermining a weighted average Z location error associated with theimaging system. The imaging method also includes selecting a thresholdvalue associated with the maximum Z location error and the weightedaverage Z location error. The imaging method further includesprescribing an N+1 sector reconstruction algorithm. If the maximum Zlocation error is less than the threshold value or the weighted averageZ location error is less than the threshold value, the imaging methodincludes performing an N+1 sector reconstruction. If the maximum Zlocation error exceeds the threshold value or the weighted average Zlocation error exceeds the threshold value, the imaging method includesprescribing an N sector reconstruction.

In a still further embodiment of the present invention, a computedtomography system includes a computed tomography scanner, a firstalgorithm operable for determining a maximum Z location error associatedwith the computed tomography system, and a second algorithm operable fordetermining a weighted average Z location error associated with thecomputed tomography system. The computed tomography system also includesa third algorithm operable for selecting a threshold value associatedwith the maximum Z location error and the weighted average Z locationerror. The computed tomography system further includes means forprescribing an N+1 sector reconstruction algorithm. The computedtomography system still further includes a fourth algorithm operablefor, if the maximum Z location error is less than the threshold value orthe weighted average Z location error is less than the threshold value,performing an N+1 sector reconstruction, and wherein the fourthalgorithm is further operable for, if the maximum Z location errorexceeds the threshold value or the weighted average Z location errorexceeds the threshold value, prescribing an N sector reconstruction.

In a still further embodiment of the present invention, an imagingsystem includes an imaging scanner, a first algorithm operable fordetermining a maximum Z location error associated with the imagingsystem, and a second algorithm operable for determining a weightedaverage Z location error associated with the imaging system. The imagingsystem also includes a third algorithm operable for selecting athreshold value associated with the maximum Z location error and theweighted average Z location error. The imaging system further includesmeans for prescribing an N+1 sector reconstruction algorithm. Theimaging system still further includes a fourth algorithm operable for,if the maximum Z location error is less than the threshold value or theweighted average Z location error is less than the threshold value,performing an N+1 sector reconstruction, and wherein the fourthalgorithm is further operable for, if the maximum Z location errorexceeds the threshold value or the weighted average Z location errorexceeds the threshold value, prescribing an N sector reconstruction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a retrospectively EKG-gatedreconstruction associated with the systems and methods of the presentinvention;

FIG. 2 is a graph illustrating the Z location error concepts associatedwith the systems and methods of the present invention;

FIG. 3 is a flow chart illustrating one embodiment of the multi-sectorback-off logic algorithm for obtaining optimal slice-sensitive CTprofiles of the present invention; and

FIG. 4 is a schematic diagram illustrating one embodiment of a computedtomography (“CT”) system incorporating the multi-sector back-off logicalgorithm for obtaining optimal slice-sensitive CT profiles of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The systems and methods of the present invention allow for the creationof relatively high temporal resolution images for cardiac applicationswhile addressing the problem of the generation of bad images due torelatively large Z location errors between sectors that are used forreconstruction. In general, the algorithm of the present invention isbased upon the measurement of maximum Z location error (“ME”) andweighted average Z location error (“WE”) and determining how far thesemeasurements are from predetermined limits.

The computation of the Z location error, ME, and WE includes a number ofsteps beginning with calculating half the detector coverage (i.e., thedistance from the center of the detector to the center of the outerrow). This is done using the following equation:half the detector coverage=[(num_rows/2)−1]*detector width.   (1)

Next, the Z location error is computed for each sector. This is done byfinding the Z location of the center view in the table space andcalculating upper (“maximum”) limit and the lower (“minimum”) limit thatthe detector may cover at this particular Z location. The maximum limitand the minimum limit are given by:maximum limit=center Z location+half the detector coverage,   (2)minimum limit=center Z location−half the detector coverage.   (3)The Z location error is computed for each sector using the followingalgorithm and is a signed value:if Z location<lower limit, Z location error=lower limit−Z location;  (4)if Z location>upper limit, Z location error=upper limit−Z location;  (5)if lower limit<Z location<upper limit, Z location error=0.   (6)

Next, the maximum error between the upper most and lower most errorsectors is calculated. This also involves calculating the maximum andminimum errors within the set of sectors and the maximum error spread.The maximum error spread is given by:maximum error spread=maximum error−minimumerror−max(Zi−Z_(desired))−min(Zi−Z_(desired)).   (7)

Next, WE is calculated using the average error weighted by the number ofviews in each sector:WE=total error over all sectors/total view over allsectors=sum(0,sector−1)|Zi−Z_(desired)|*Wi.   (8)

The percentage of image locations, or images, that fall into the gap isgiven by gap/(gap+overlap).

Referring to FIG. 1, in one embodiment of the present invention, aretrospectively EKG-gated reconstruction is illustrated. Theretrospectively EKG-gated reconstruction provides a plurality of imagelocations 10 that vary as a function of Z location associated withpredetermined points along an EKG cycle 12 that vary as a function oftime. The predetermined points along the EKG cycle 12 include, forexample, a first cycle 14, a second cycle 16, a third cycle 18, and afourth cycle 20. The reconstruction algorithm of the present inventionprovides a continuous view stream 22 consisting of a plurality of viewregions 24 utilized by the reconstruction algorithm. These view regions24 correspond to the first cycle 14, the second cycle 16, the thirdcycle 18, and the fourth cycle 20. A plurality of detector rows 27 areused to obtain images as part of a low-pitch helical scan 26.

In another embodiment of the present invention, the Z location errorconcepts described above are illustrated in FIG. 2. FIG. 2 shows aplurality of sectors, including a sector N−1 30, a sector N 32, and asector N+1 34. Each sector includes a tolerance level 36. The Z locationfor a given image 38 and a Z location error >0 are also shown. Further,the half detector coverage 42 (i.e., 1.5 detector for a 4-rowconfiguration, 3.5 detector for an 8-row configuration, 7.5 detector fora 16-row configuration) and the range 44 are also shown.

As described above, the multi-sector back-off logic algorithm forobtaining optimal slice-sensitive CT profiles of the present inventionis based upon deciding the maximum number of sectors to reconstruct in agiven situation. This determination is made based upon how far two givensectors are separated with respect to the Z location. The algorithmbegins with a predetermined number of sectors and, based upon themaximum Z location error and the weighted average Z location error,backs off to a lesser number of sectors until images may be generatedwith minimum error. This algorithm is illustrated in FIG. 3.

Referring to FIG. 3, in a further embodiment of the present invention,the multi-sector back-off logic algorithm for obtaining optimalslice-sensitive CT profiles of the present invention 50 begins with the“auto burst” algorithm 50 trying an N+1 or N sector reconstructionalgorithm 52,56. For example, a user may prescribe a four sectorreconstruction 54 and the auto burst algorithm 50 may try a four sector(N sector) reconstruction algorithm 56. If ME is less than the thresholdor WE is less than the threshold 58, then a four sector reconstructionis performed 60. If ME exceeds the threshold or WE exceeds the threshold58, then the auto burst algorithm 50 tries a three sector (N−1 sector)reconstruction algorithm 62. This is also the starting point if the userprescribes a three sector reconstruction 64. If ME is less than thethreshold or WE is less than the threshold 66, then a three sectorreconstruction is performed 68. If ME exceeds the threshold or WEexceeds the threshold 66, then the auto burst algorithm 50 tries a twosector (N−2 sector) reconstruction algorithm 70. This is also thestarting point if the user prescribes a two sector reconstruction 72. IfME is less than the threshold or WE is less than the threshold 74, thena two sector reconstruction is performed 76. If ME exceeds the thresholdor WE exceeds the threshold 74, then the auto burst algorithm 50performs a single sector reconstruction 78 (i.e., a snapshot segment).

Referring to FIG. 4, in a still further embodiment of the presentinvention, a CT system 80 incorporating the multi-sector back-off logicalgorithm for obtaining optimal slice-sensitive CT profiles 50 includesa CT scanner 82 coupled to a data acquisition/control and imagegeneration sub-system 84. Preferably, the CT scanner 82 is also coupledto an EKG monitor 86 or the like operable for measuring R-peak events orthe like. The data acquisition/control and image generation subsystem 84may be operable for performing, for example, an EKG-gated cardiacreconstruction. In order to do this, the data acquisition/control andimage generation subsystem 84 includes a real-time control/datacollection algorithm 88, the auto burst algorithm 50, and an imagegeneration algorithm 90. The data acquisition/control and imagegeneration subsystem 84 is operable for transmitting an image stream toan operator's console 92 or the like including a network component 94, afilming component 96, an archive component 98, an exam prescriptioncomponent 100, and a visualization component 102. The exam prescriptioncomponent 100 and the visualization component 102 may be associated witha prescription display CRT 104 or the like. The operator's console 92 iscoupled to a review/analysis workstation 106 also including a networkcomponent 108, a filming component 110, and an archive component, aswell as an image review component 114.

It is apparent that there has been provided, in accordance with thesystems and methods of the present invention, a multi-sector back-offlogic algorithm for obtaining optimal slice-sensitive CT profiles.Although the systems and methods of the present invention have beendescribed with reference to preferred embodiments and examples thereof,other embodiments and examples may perform similar functions and/orachieve similar results. All such equivalent embodiments and examplesare within the spirit and scope of the present invention and areintended to be covered by the following claims.

1. A computed tomography method, comprising: determining a maximum Zlocation error; determining a weighted average Z location error;selecting a threshold value associated with the maximum Z location errorand the weighted average Z location error; prescribing an N+1 sectorreconstruction algorithm; if the maximum Z location error is less thanthe threshold value or the weighted average Z location error is lessthan the threshold value, performing an N+1 sector reconstruction; andif the maximum Z location error exceeds the threshold value or theweighted average Z location error exceeds the threshold value,prescribing an N sector reconstruction.
 2. The computed tomographymethod of claim 1, further comprising, if the a second maximum Zlocation error is less than the threshold value or the a second weightedaverage Z location error is less than the threshold value, performing anN sector reconstruction.
 3. The computed tomography method of claim 2,further comprising, if the second maximum Z location error exceeds thethreshold value or the second weighted average Z location error exceedsthe threshold value, prescribing an N−1 sector reconstruction.
 4. Thecomputed tomography method of claim 1, wherein the computed tomographymethod is used to perform cardiac imaging.
 5. A computed tomographymethod for obtaining optimal slice-sensitive profiles, comprising:determining a maximum Z location error associated with a computedtomography system; determining a weighted average Z location errorassociated with the computed tomography system; selecting a thresholdvalue associated with the maximum Z location error and the weightedaverage Z location error; prescribing an N+1 sector reconstructionalgorithm; if the maximum Z location error is less than the thresholdvalue or the weighted average Z location error is less than thethreshold value, performing an N+1 sector reconstruction; and if themaximum Z location error exceeds the threshold value or the weightedaverage Z location error exceeds the threshold value, prescribing an Nsector reconstruction.
 6. The computed tomography method of claim 5,further comprising, if the a second maximum Z location error is lessthan the threshold value or the a second weighted average Z locationerror is less than the threshold value, performing an N sectorreconstruction.
 7. The computed tomography method of claim 6, furthercomprising, if the second maximum Z location error exceeds the thresholdvalue or the second weighted average Z location error exceeds thethreshold value, prescribing an N−1 sector reconstruction.
 8. Thecomputed tomography method of claim 5, wherein the computed tomographymethod is used to perform cardiac imaging.
 9. An imaging method forobtaining optimal slice-sensitive profiles, comprising: determining amaximum Z location error associated with an imaging system; determininga weighted average Z location error associated with the imaging system;selecting a threshold value associated with the maximum Z location errorand the weighted average Z location error; prescribing an N+1 sectorreconstruction algorithm; if the maximum Z location error is less thanthe threshold value or the weighted average Z location error is lessthan the threshold value, performing an N+1 sector reconstruction; andif the maximum Z location error exceeds the threshold value or theweighted average Z location error exceeds the threshold value,prescribing an N sector reconstruction.
 10. The imaging method of claim9, further comprising, if the a second maximum Z location error is lessthan the threshold value or the a second weighted average Z locationerror is less than the threshold value, performing an N sectorreconstruction.
 11. The imaging method of claim 10, further comprising,if the second maximum Z location error exceeds the threshold value orthe second weighted average Z location error exceeds the thresholdvalue, prescribing an N−1 sector reconstruction.
 12. The imaging methodof claim 9, wherein the computed tomography imaging method is used toperform cardiac imaging.
 13. A computed tomography system, comprising: acomputed tomography scanner; a first algorithm operable for determininga maximum Z location error associated with the computed tomographysystem; a second algorithm operable for determining a weighted average Zlocation error associated with the computed tomography system; a thirdalgorithm operable for selecting a threshold value associated with themaximum Z location error and the weighted average Z location error;means for prescribing an N+1 sector reconstruction algorithm; a fourthalgorithm operable for, if the maximum Z location error is less than thethreshold value or the weighted average Z location error is less thanthe threshold value, performing an N+1 sector reconstruction; andwherein the fourth algorithm is further operable for, if the maximum Zlocation error exceeds the threshold value or the weighted average Zlocation error exceeds the threshold value, prescribing an N sectorreconstruction.
 14. The computed tomography system of claim 13, whereinthe fourth algorithm is further operable for, if the a second maximum Zlocation error is less than the threshold value or the a second weightedaverage Z location error is less than the threshold value, performing anN sector reconstruction.
 15. The computed tomography system of claim 14,wherein the fourth algorithm is further operable for, if the secondmaximum Z location error exceeds the threshold value or the secondweighted average Z location error exceeds the threshold value,prescribing an N−1 sector reconstruction.
 16. The computed tomographysystem of claim 13, wherein the computed tomography system is used toperform cardiac imaging.
 17. An imaging system, comprising: an imagingscanner; a first algorithm operable for determining a maximum Z locationerror associated with the imaging system; a second algorithm operablefor determining a weighted average Z location error associated with theimaging system; a third algorithm operable for selecting a thresholdvalue associated with the maximum Z location error and the weightedaverage Z location error; means for prescribing an N+1 sectorreconstruction algorithm; a fourth algorithm operable for, if themaximum Z location error is less than the threshold value or theweighted average Z location error is less than the threshold value,performing an N+1 sector reconstruction; and wherein the fourthalgorithm is further operable for, if the maximum Z location errorexceeds the threshold value or the weighted average Z location errorexceeds the threshold value, prescribing an N sector reconstruction. 18.The imaging system of claim 17, wherein the fourth algorithm is furtheroperable for, if the a second maximum Z location error is less than thethreshold value or the a second weighted average Z location error isless than the threshold value, performing an N sector reconstruction.19. The imaging system of claim 18, wherein the fourth algorithm isfurther operable for, if the second maximum Z location error exceeds thethreshold value or the second weighted average Z location error exceedsthe threshold value, prescribing an N−1 sector reconstruction.
 20. Theimaging system of claim 17, wherein the imaging system is used toperform cardiac imaging.
 21. The computed tomography method of claim 1wherein: determining the maximum Z location error further comprisesdetermining a first maximum Z location error and a second maximum Zlocation error; and determining the weighted average Z location errorfurther comprises determining a first weighted average Z location errorand a second weighted average Z location error.
 22. The computedtomography method of claim 5 wherein: determining the maximum Z locationerror further comprises determining a first maximum Z location error anda second maximum Z location error; and determining the weighted averageZ location error further comprises determining a first weighted averageZ location error and a second weighted average Z location error.
 23. Theimaging method of claim 9 wherein: determining the maximum Z locationerror further comprises determining a first maximum Z location error anda second maximum Z location error; and determining the weighted averageZ location error further comprises determining a first weighted averageZ location error and a second weighted average Z location error.
 24. Thecomputed tomography system of claim 13 wherein: the first algorithm isfurther operable for determining a first maximum Z location error and asecond maximum Z location error; and the second algorithm is furtheroperable for determining a first weighted average Z location error and asecond weighted average Z location error.
 25. The imaging system ofclaim 17 wherein: the first algorithm is further operable fordetermining a first maximum Z location error and a second maximum Zlocation error; and the second algorithm is further operable fordetermining a first weighted average Z location error and a secondweighted average Z location error.
 26. An imaging apparatus comprising:an imager; and a computer programmed to: acquire scan data; select apredetermined number of sectors corresponding to the scan data;determine a multiple-sector Z location error corresponding to thepredetermined number of sectors for a desired Z location; select a Zlocation error threshold; reconstruct an image from less than thepredetermined number of sectors if the multiple-sector Z location erroris above the Z location error threshold; otherwise reconstruct an imagefrom the predetermined number of sectors.
 27. The imaging apparatus ofclaim 26 wherein the computer is further programmed to: determine a Zlocation for each of the predetermined number of sectors; determine adetector coverage associated with the imager; calculate an upper limitand a lower limit of the detector coverage for each of the predeterminednumber of sectors; and determine a single-sector Z location error foreach of the predetermined number of sectors based on the respective Zlocation, the upper limit, and the lower limit of each sector.
 28. Theimaging apparatus of claim 27 wherein the computer is further programmedto: set the single-sector Z location error for each sector equal to thelower limit minus the Z location, if the respective single-sector Zlocation is less than the lower limit; set the single-sector Z locationerror for each sector equal to the upper limit minus the Z location, ifthe respective single-sector Z location is greater than the upper limit;otherwise set the single-sector Z location equal to zero.
 29. Theimaging apparatus of claim 27 wherein the computer is further programmedto: identify an upper-most Z location error sector of the predeterminednumber of sectors based on the single-sector Z location error of eachsector; identify a lower-most Z location error sector from thepredetermined number of sectors based on the single-sector Z locationerror of each sector; and determine the multi-sector Z location error bycalculating a maximum Z location error based on the upper-most Zlocation error sector and the lower-most Z location error sector. 30.The imaging apparatus of claim 29 wherein the computer is furtherprogrammed to calculate the maximum Z location error in accordance with:maximum _(—) error _(—) spread=maximum _(—) error−minimum _(—) errorwhere: maximum _(—) error represents the single-sector Z location errorcorresponding to the upper-most Z location error sector and minimum _(—)error represents the single-sector Z location error corresponding to thelower-most Z location error sector.
 31. The imaging apparatus of claim29 wherein the computer is further programmed to determine themultiple-sector Z location error by calculating a weighted average Zlocation error based on the upper-most Z location error sector and thelower-most Z location error sector.
 32. The imaging apparatus of claim31 wherein the computer is further programmed to calculate the weightedaverage Z location error in accordance with: WE=total error over allsectors/total view over all sectors where: WE represents the weightedaverage Z location error and total error over all sectors represents atotal Z location error over all sectors.
 33. An imaging methodcomprising: accessing a predetermined number of sectors to reconstruct;receiving scan data associated with the predetermined number of sectors;determining a Z location error threshold; determining a plurality of Zlocations for a desired Z location corresponding to the predeterminednumber of sectors; calculating a first multi-sector Z location errorbased on the plurality of Z locations; reconstructing less than thepredetermined number of sectors to create an image if the firstmulti-sector Z location error is above the Z location error threshold;otherwise reconstructing the predetermined number of sectors to createan image.
 34. The method of claim 33 further comprising: determining aplurality of detector coverage limits, each detector coverage limitcorresponding to a respective one of the predetermined number ofsectors; determining a plurality of single-sector Z location errorsbased on the plurality of detector coverage limits; and calculating thefirst multi-sector Z location error based on the plurality ofsingle-sector Z location errors.
 35. The method of claim 33 whereinreconstructing less than the predetermined number of sectors furthercomprises: calculating a second multi-sector Z location error based onthe plurality of Z locations; and reconstructing a set of sectors havingone less sector than the predetermined number of sectors if the secondmulti-sector Z location error is below the Z location error threshold.36. The method of claim 33 wherein calculating the first multi-sector Zlocation error comprises calculating a maximum Z location error and aweighted average Z location error based on the plurality of Z locations.37. The method of claim 33 wherein calculating the maximum Z locationerror comprises determining a maximum error between an upper most sectorand a lower most sector of the predetermined number of sectors.
 38. Themethod of claim 36 wherein calculating the weighted average Z locationerror comprises determining an average Z location error weighted by atotal view of the predetermined number of sectors.